1. Field of the Invention
The present invention relates to watch exterior parts having complicated shapes and formed of cemented carbide or stellite alloy and further relates to manufacturing methods thereof.
2. Description of the Background Art
Recently, hard materials which are damage resistant and have excellent durability have come into use for watch exterior parts such as watch frames, watch band pieces and the like. Especially, there is a tendency that cemented carbides including WC, TaC, TiC and so forth, or stellite alloys containing Co--Cr--W, for example, are widely used.
Such cemented carbides or stellite alloys have a texture in which hard particles such as carbide, nitride or carbonitride of W, Ta, Ti, Cr, for example, are bonded with metal of the iron group such as Co, Fe, Ni and the like, and are manufactured by conventionally well-known powder metallurgy. That is, they are manufactured by a method of mixing WC powder, TaC powder, Co powder, Ni powder etc. according to a predetermined alloy composition, molding the material powder of mixed alloy composition by pressing the powder, and sintering the obtained molded bodies.
However, since molded bodies are obtained by pressing in the above-described normal powder metallurgy method there have been problems such as limitation of the shapes of manufactured products and dimensional precision, for example. That is, products having shapes that can be formed in one axis direction only can be manufactured by die compaction. Even if the CIP (Cold Isostatic Press) technique capable of forming three-dimensional shapes is used, excellent precision cannot be expected because the products are molded inside rubber molds. Accordingly, watch exterior parts have been conventionally obtained by manufacturing sintered bodies having simple shapes by the normal powder metallurgy method, performing secondary machining operations on the sintered bodies to realize complicated shapes such as various kinds of watch frames and watch band pieces having three dimensional curved surfaces and small holes, and performing surface finishing by polishing in order to improve the decorativeness of portions such as surfaces of exterior parts as needed.
However, because cemented carbide and stellite alloys are extremely difficult to be machined, they can be machined only by grinding with diamond grinding stones or by performing discharge machining as a secondary operation. Particularly, discharge machining operation have been essential in forming three-dimensional curved surfaces on inner surfaces of watch frames and interior surfaces of small holes for provision of stems, and the like. However, if sintered bodies of cemented carbide or stellites alloys are subjected to a discharge process, the corresponding processed surfaces are embrittled over a depth of approximately 5 through 100 .mu.m due to the removal, oxidation or the like of metallic constituents. Such embrittlement decreases the material strength, which is likely to cause minor cracking, chipping or breaking of the processed surface due to external impacts leading to breakage of the entirety of a watch exterior part.
Accordingly, watch exterior parts formed of cemented carbide or alloy corresponding to stellite have been conventionally made with simple shapes to reduce the need for discharge machining operations as much as possible. Alternatively, the thickness of such parts has been designed to be larger than needed in order to maintain the strength of the parts after a discharge operation. Therefore conventional methods have disadvantages such as the limitations of designs of watch frames and watch bands, and the increase in the total weight of a watch, for example.